Method and apparatus for the rectification of alternating currents



Febo 8 l c. G. SMITH METHOD AND APPARATUS FOR THE RECTIFICA'IION OF ALTERNATING CURRENTS Filed Oct. 8 1920 LOAD Patented Feb. s, 1927.

UNITED STATES.

' PATENT oFFlcvE. 'l

CHARLES G. SMITH, OF MEDFORD, MASSACHUSETTS, IASSIG'NOR, BY MESNE IASSIGN- MENTS, TO RAYTHEON MANUFACTURING COMPANY, OF SETTS, A CORPORATION OF MASSACHUSETTS.

CAMBRIDGE, MASSACHU- METHOD AND APPARATUS FOR THE RECTIFICATION OF ALTERNATING CURRENTS. y

Application filed October 8, 1920. Serial No. 415,536.

The present invention relates to a method and apparatus for the rectification of alternating currents.

In my copending Patent No. 1,545,207 granted July 7, 1925, I have disclosed a new form of insulating gap for preventing conduction through a gas. According to this disclosure, the surfaces ofthe electrodes immersed in the gas are located in close proximitv to normally prevent a sufiicient number of collisions between electrons and atoms to initiate gaseous conduction. As explained therein, this result is accomplished by spacing the active surfaces of the electrodes at different potentials a distance apart which is relatively short and of the order of magnitude of the mean free path of the electrons.

I have discovered in connection with an apparatus of this character -that it is possible to render the gap between electrodes, /substantially freely conducting in one direction and nonconducting up to voltages which will actually7 destroy the tube in the opposite direction. n the simplest and most efiicient embodiment of the invention which has yet been devised asymmetric electrodes are employed in conjunction with a constant magnetic field. Although this .magnetic field should be constant yet it is found that between certain critical limits the tube is conducting in a direction which renders thc outer electrode negative and nonconducting in the opposite direction. Although the critical limits of field strength are not accurately determined, it is known that for a certain magnetic field strength above a minimum value the tube is conducting in one direction only up to a certain higher critical value at which the tube conducts freely in both directions. In the specific embodiment of the invention disclosed'comprising concentric cylinders of different diameters it has been determined that the critical field strength at which the tube becomes conducting in both directions bears a certain definite relation to the minimum value of field strength at which the tube rectifies which is proportional to the diameters of the inner and outer cylinders. In one embodiment of the invention in whichthe diameters of the inner and outer cylinders were-as one to two, the same relation prevailed between the lower and upper limits of field strength boundingthe region in which the-tube was conducting in one direction only.

It is believed that the theory underlying this mode of operation of the tube is some'- what as follows: Owing to the magnetic field, electrons leaving the outer or larger cylinder are caused to describe curves with# out coming immediately into contact with the inner cylinder, thus affording opportunity for collision, whereas the paths of electrons leaving the inner cylinder arenotdistorted or are distorted insufficiently to prevent these electrons from immediately reaching the outer cylinder. They therefore reach the outer cylinder after having traversed only a short path. According to this theory, gaseous conduction is permitted in a direction which makes the outer cylinder negative.

The advantages `of this construction when employed as a rectifier of high voltages are manifold as a large amount of energy may be transmitted through a gaseous medium byv ionic conduction and the voltage drop through the tube for potentials of the order of ten thousand volts or greater is negligible. Further more the tube is not injured by short circuits of short duration.

One feature of the invention accordingly contemplates the use of two electrodes immersed in a gas vand having similarly curved surfaces together with means for creating a magnetic field of constant strength in the space between the electrodes to cause the tube to conduct freely in one direction and insulate in the opposite direction.

A further feature of thelinvention contemplates the employment of asymmetric electrodes immersed in gas together with a constant magnetic field extending substantially axially of the electrodes.

In the accompanying drawings illustrat`l ing the preferred formof the invention Fig. l represents a section in elevation of a tube adapted to act as a rectifier of high voltages; and Fig. 2 represents a typical circuit in which the` rectifier tube may be embodied.

Upon referring to the illustrated embodiment of the invention as shown in Fig. 1 it Will be observed that ak sealed enclosing vessel 10 has contained therein two concentric electrodes 12 and 13 which are con-r structed of non-magnetic material such as molybdenum. The opposite surfaces of the two electrodes are subjected to a potential difference and when the tube is operating conduction takes place in one direction in the space separating these surfaces. The active surfaces of the electrodes are located in suiiiciently close proximity `to normally prevent gaseous conduction through the space separating them even under excessively high potentials (the gas preferably being rarefied), as described in my cepending patent hereinbefore referred to. By inter-posing a constant magnetic eld with lines of force extending substantially parallel to the axes of the electrodes, the tube is made conducting in one direction and insulating in the opposite direction.. This magnetic field may be created by a permanent magnet 15 located with respect to the tube as shown in Fig. 1. Cooperating with the permanent magnet are tubes or cylinders 17 and 18 of magnetic material which engage with opposite ends of the electrode cylinder 13 and serve to localize the magnetic field in the region between the electrodes 12 and 13. The two cylinders 17 and 18 are clamped to the electrode cylinder 13 and to one another through a sleeve 19 of nonmagnetic material. It will be observed that the outer electrode 13 consists of a thin metallic cylinder having its opposite ends received in grooves formed in the adjacent faces of the cylinders 17 and 18. In order to permit the free escape of heat from the region surrounding the outer electrode 13, the connector sleeve 19 is preferably provided with openings or slots 14 asdindicated in the drawings. The inner electrode l2 is connected at its opposite ends with iron rods 20 and 21, the rod 20 having a head 22 formed thereonv and the rod 21 having its end threaded at 23. The electrode is preferably fused or welded to the rods to form a continuous rod having its central section of non-magnetic material. Surrounding the outer .ends of the rods 20 and 21 are insulating cylinders 24 and 25 of glass or similar material which are seated in recesses 26 formed respectively in the cylinders 17 and 18. The insulating cylinders may be heldin place by collars 27 and 28 of copper or other non-magnetic: material, each having a neck 29 which projects into the space between the rods 20 and 21 and the surrounding cylinders 24 and 25. The several elements are retained 'in assembled relation through the provision of a nut 30 engaging with the threaded end 23 and serving to draw t-he collars- 27 cand 28 against the ends of the cylinders 24 and 25. In order to insure that all paths through the supporting dielectric subjected to a potential gradient shall be long and that all paths through the gas shall be short the outer ends of the cylinders 17 and 18 are counter- .20 and 2l.

bored at. 3l to separate the point otvontaet. between the meta-l and insulating cylinders a substantial distance from the collars :7 and 28 which are in contact with theA rods provided with an annular tiange 33 extendiing parallel lult .spaced away t rom the walls of the enclosing vessel. Froml an inspection of the drawings, it will be observed that the outer cylinders d o not contact directly with the enclosing vessel but are centered within the vessel and spaced away from the walls by rings or washers 35. One of the leadingin wires 36 may be conveniently connected with the head 22, passing outwardly-through the end of the tube and the other wire 37 may be connected with the sleeve 19 passing outwardly through the side of the tube at 38.

From the foregoing `it will be evident that gaseous conduction around the boundary of the space between t-he cathode 13 and anode 12, that is from the anode around the end of the cylindrical space between cathode and anode to the outside of the cathode, is obstructed by the pole pieces 17 and 18. lVllile each pole ieee is spaced from the anode this space 1s comparable to the mean free path of electrons in the gas (being somewhat less than the spacing between cathode and anode) and therefore just as eHective to obstruct conduction as if the gap were wholly closed as between the cathode and pole pieces.

It will also be evident that the space between the cathode and anode is rendered alternately conducting and non-conducting as a result of variations in the state of ionization of the gas in the space, these variations being efected in the illustrated embodiment by periodic changes in the relationship between the electric and magnetic fields; that is, while one of the fields (the magnetic field) is constant the other field (the electric field) varies, both in intensity and direction, at the frequency of the alternating field impressed upon the electrodes. Thus the discharge path is selectively controlled in accordance with the instantaneous polarity of the electrodes; when the cathode is negative current fiows whereas when the anode is negative little if any current flows. Obviously this invention is particularly applicable to glow-discharge tubes in contradistinction to arcing tubes.

The voltage drop through the tube even when employing air approximates only about 300 volts which is negligible when voltages of the order of 10,000 volts or greater are being rectied. It should be understood in this connection however, that inasmuch as the voltage drop through the tube approximates at the most about 300 volts, the apparatus may probably be used to advantage in the rectification of alternating currents where a potential of 500 volts In addition the collars are cach f" or even less is employed. By using helium or a similar gas witlun the enclosing vessel,

this voltage drop through the tube is greatly reduced, the use of such a gas cutting-the voltagel drop practically in half. In addition the use ot' helium is advantageous as it has a small molecule and in consequence a; long lnean free-path which permits the use of an operating pressure for the gas considerably greater than the practical operating pressure for air.

Although the exact field strength for dif- Jferent types and dimensions of tubes has not been accurately determined, it is lknown that in one case a field strength of two hundred gauss caused the tube to rectify potentials as high as 20.000 volts and that a decrease of twenty-five gaurs caused the tube to become. insulating.

In the circuit. diagram shown in Fig. 2, an input circuit is indicated at 40 capable of delivering alternating current at 110 volts to the primary of a step-up transformer 42. The secondary of this transformer is embodied in an output circuit 43 containingthe tube which rectifies the high voltage alternating current delivered thereto by the secondary of the transformer. The output circuit' obviously may be connected with any desired form of load circuit (not shown) to utilize the high voltage current so rectified.

It will be obvious to those skilled in the art that the rectilier tube, if so desired, may

be made with two parts to rectify both halves of the alternating current wave, thus increasing the applicability of the tube. This form of tube may be advantageously employed in the production of high voltage direct current for X-ray apparatus, electrical precipitation of suspended particles by the Cottrell process, railway electritication, and other purposes where stationary rectifiersor rotary converters are now employed.

From the foregoing it will be evident that the sleeve 25 of insulating material, which surrounds the conductor to electrode 12 between this conductor and the conductor 18 which contacts with electrode 13, as well as the thin metallic cup-shaped plate 27, which extends from the end of the sleeve outwardly to the region of the surrounding tube, serves to restrain gaseous conduction between said conductors and to confine the electrical discharge to the interior of the hollow electrode. Insulating sleeve 24 and cupped plate 28 perform similar functions at the upper end of the tube.

ly virtue of the narrow space between conductor 20 the member 18 (also 21 and 17). which spacing is less than the mean free path of electronsin the gas, and the insulating ring 25 (also 24), which is similarly spaced from the conductor, and the plate 27 (also 28) which is similarly spaced from the tube wall, the gaseous discharge betw en the tube is protected from deleterious eti'ects of the discharge such as interaction between the gas and the glass when radiation from thel discharge reaches the region of thetube wall as in prior devices.

In the particular embodiment of the, in-

vention illustrated in the drawing, the means v additional to the space charge (charged particles) for selectively causing electrons passing -in different Adirections through the gas to take paths of widely different lengths comprises the magnetic field.

In a. generic sense the term hollow cathode includes a cathode whose active` surface only partially surrounds a gaseous medium so that the medium immediately adjacent the active surfacelis more or less pocketed or confined.

I claim- 1.` A method' of controlling the flow, of electric currents through a gas lilled space which consists in positioning two electrodes 4with their opposing surfaces in such close proximity as to normally prevent conduction through the space, and lengthening the paths of electrons coming from one of the electrodes suliiciently to cause gaseous conduction in one direction through the space.

2. An electrical apparatus comprising concentric electrodes of. non-magnetic material, a gas filled vessel enclosing the electrodes, cylinders of magnetic material positioned at opposite ends of the outer electrode, and

`means for interposing a magnetic eld in the space 'separating the electrodes, the region of which is determined by the cylinders of magnetic material.

3. An electrical apparatus comprising asymmetric electrodes immersed in a gas with a space therebetween comparable to the mean free path of electrons in the gas, Iand means for causing gaseous ionization and conduction for electrical potentials in one direction only between the electrodes.

4. An electrical apparatus comprising concentric electrodes of non-magnetic material, positioning members of magnetic material engaging with oppositel ends of the electrodes, means. for holding the parts in assembled relation and a gas filled vessel enclosing the electrodes and retaining members.

5. A rectifier comprising a gas filled receptacle, means for creating an electric field in -a portion of the gaseous region within the receptacle, means for limiting the flights of electrons through the gas in one direction of applied electric. field to 'distances which are short and comparable to the mean free path of electrons in the as, and means whereby electrons impelled y a fieldin the reverse direction take paths sufiiciently long to cause gaseous ionization and conduction.

6. An electrical apparatus comprising a gas filled tube, inner and outer cylindrical electrodes received within the tube and located with their axes substantially coincident, means for creating an electric field in the space separating the inner and outer electrodes, cylinders of magnetic material positioned concentric with the electrodes, and means cooperating with the cylinders of magnetic material for introducing a magnetic field into the annular space separating the electrodes.

7. In a rectifier, a vacuum tube, a hollow cathode and an anode arran ed opposite thereto at a distance less than t e mean free path required for ionization under electromotive forces impressed in one direction.

8. The method of rectifying alternating current which comprises impressing an alternating electric field in a gas space which is suticiently short and in which the gas pressure is suiiciently low to prevent initiation of substantial conduction directl across said space in one direction by Va fiel, strong 'enough to initiate conduction across longer spaces, and causing gaseous ionization and conduction to take place in the opposite direction in response to said alternating electric field. y

9. The method of rectifying alternating current which comprises impressing an alternatingelectric field in a gas space which is sufiiciently short and in which the gas Cil pressure is sufiiciently low to prevent initiation of substantial conduction directly across said space in one direction by a field strong enough to initiate conduction-across longer spaces, causing gaseous ionization and conduction to take place in the opposite direction in response to Said alternating electric field, and obstructing gaseous conductionA around the boundary of said space.

10. A gaseous conduction device comprising a. hollow cathode, an anode presented to the interior surface of the hollow cathode, and a metallic obstruction for preventing discharge from the anode to the outside of the cathode, the obstruction being s aced sufiiciently close to one of the electro es to prevent substantial ionization in the space by electrons passing across the space.

11. A gaseous conduction device comprising a hollow cathode, and means to control the rate of electron emission from the cathode by varying the state .of ionization of the gas inside the hollow cathode.

12. A gaseous conduction device comprising ahollow cathode, and means to vary electron emission from the inner surface of the cathode by recurrently ionizing the gas in the hollow cathode ata predetermined frequency.

13. The method of producing pulsating gaseous conduction between two electrodes with a gas space therebetween suiciently short normally Ato prevent conduction therebetween, which comprises concomitantly im pressing electric and magnetic fields upon the space between the electrodes and periodicall varying the relationship between the fiel s to vary the conductivity of the gas in the space.

14. The method of producing pulsating gaseous conduction between two electrodes with a gas space therebetween suficiently short normally to prevent conduction, which comprises impressing an electric field' between the electrodes and selectively controlling the discharge path depending upon the instantaneous polarity of the electro es 15. A gaseous conduction device comprising a hollow cathode, an anode presented to the interior surface of the cathode, means for producing discharge between the anode and the interior surface of the cathode, and means for preventing discharge between the anode and the exterior of the cathode.

16. Electrical apparatus comprising opposed elect-rodes having therebetween a gas space sufiiciently short to prevent substantial ionization `by electrons traveling directly across the space and means permitting electrons to follow longer paths in one direction of applied potent-ial between the electrodes to initiate gaseous conduction without initiating substantial conduction when the same potential is applied in the opposite direction.

17. An electrical discharge tube containing a hollow electrode, a conductor extending through the wall of the tube, an electrode' surface presented to the interior of said-hollow electrode and receiving current through said conductor, and a metallic member surrounding said conductor intermediate said surface and that part of said wall through which said conductor extends, said member extending from the region of said conductor to the region of said wall.

18. An electrical discharge tube containing a hollow electrode, a conductor extending through the wall of ythe tube, an electrode surface presented to the interior of said hollow electrode and receiving current through said conductor. an insulating sleeve surrounding said conductor intermediate said surface and that part of said wall through which said conductor extends, and a metallic member surrounding said conductor intermediate said sleeve and said part of the tube Wall.

19. Electrical discharge apparatus comprising electrodes separated by a gaseous medium, means for permitting a discharge between the electrodes through the aseous medium and means for confining tie discharge to the electrodes, including surfaces spaced apart a distance which while appreciable is substantially confined to the mean free path of the electrons in the gas.

20. An electrical discharge tube having interior electrode surfaces presented to each other across an intervenino space a conductor extending through 2the wa of the tube for conducting current to one of said surfaces, and a metallic member surrounding said conductor intermediate said-surfaces and that'part of the tube wall through which said conductor extends, said member being in non-conducting relation to said one surface.

21. Electrical discharge apparatus comprising electrodes separated by a gaseous medium, means for permitting a discharge between the electrodes through the gaseous medium, and means for confining the discharge to the electrodes, including surfaces spaced apart a distance which while appreciable is substantially confined to the mean free path of electrons in the gas, one of the surfaces being insulating. Y

, 22. An electrical discharge tube containing a ,hollow electrode having "an opening in one end,.an electrode surface presented to the interior of said hollow electrode, a conductor passing through the wall of the tube for conducting current to said surface, a second conductor in conducting relation to said hollow electrode, and a sleeve of insulating material surrounding the first conductor with an outer peripheral surface between the two conductors.

23. An electrical discharge tube containing electrodes separated by a gaseous medium, means permitting aseous discharge between the electrodes, an means presented to the surface of one of the electrodes throughout the entire circumference thereof, wlth a surface opposed thereto across a asace which while appreciable is substanti y confined 'to the mean free path of electrons in the gaseous medium, whereby the electrical discharge is confined to the area of the latter electrode which is presented to the other electrode.

24. An'electrical discharge tube containing an electrode having an extensive discharge surface, a smaller active surface pre-.

sentedto said discharge surface across a gas space, a conductor extending through the wall of said tube for conducting current to said active surface, and au annular member surrounding said conductor intermediate said active surface and said wall with a surface opposed thereto across a ace which while appreciable is sufficiently s ort to prevent the discharge from passing thereinto. l

25. An electrical discharge tube contain'- ing an electrode having an extensive discharge surface, a smaller active surface presented to said discharge surface across a gas space, a conductor extending through the wall of said tube for conducting current to said active surface, and an annular metallic member surrounding said conductor adjacent said active surface with a surface opposed thereto across a space which while appreciable is sufliciently short to prevent the discharge from passing thereinto.

246. An electrical dischargetube containing an electrode having an extensive wdischarge surface, a smaller active surface presented to said discharge surface across a gas space, a conductor extending through the wall of said tubey for conducting current to said active surface, and means for confining the discharge to said active surface including a ring of insulation surrounding said conductor, said ring having a surface opposed to the conductor across a space which while appreciable is sufficiently short to prevent the discharge from passing thereinto.

27. Electrical discharge apparatusv comprising a tube containin electrode surfaces separated by gaseous me ium, and means for shielding the entire wall of the tube from light radiation emanating from the electrical discharge between said surfaces, said means including a chamber surrounding'said surfaces, an interior face of said chamber constituting one of said electrode surfaces.

28. A gaseous conduction rectifier-comprising a gas filled receptacle, a pair of electrodes received in the receptacle, means for permitting the formation of a positivefspace charge adjacent one of the electrodes, and means additional to the space charge for selectively causing electrons passing in opposite directions between the electrodes, by virtue of alternating potential applied to the electrodes, to take paths of widely different lengths.

29. A gaseous conduction rectifier comprising two electrodes having surfaces of different areas .presented to each other, means for preventing substantial discharge between the electrodes, during the half-cycles of applied alternating potential when the smaller surface is negative, including a gas filled gap sufficiently short normally to prevent gaseous conduction thereacross, and means whereby the electrons coming from the larger surface, during alternate halfcycles of applied alternating potential when the latter surface is negative, take paths sufficiently long to cause gaseous conduction during said alternate half-cycles.

.CHARLES G. SMITH. 

